[metadata] Mark several functions external only (#6384)

[metadata] Mark several functions external only (#6384)

* [runtime] Mark mono_class_get and mono_class_get_full external only.

   Runtime should use mono_class_get_checked and
   mono_class_get_and_inflate_typespec_checked, respectively.

* [runtime] Mark mono_ldtoken external only

   Runtime should use mono_ldtoken_checked

* [metadata] Mark mono_type_create_from_typespec external only

   Runtime should use mono_type_create_from_typespec_checked

* [runtime] Remove mono_class_inflate_generic_method_full.

   Mark mono_class_inflate_generic_method external only.

   Runtime should use mono_class_inflate_generic_method_full_checked.

* [runtime] Mark mono_get_inflated_method external only

   This function is obsolete.  It should have no callers.

* [metadata] Mark mono_field_from_token external only

   Runtime should use mono_field_from_token_checked

* [runtime] Add MonoError arg to mono_class_get_inflated_method

* [metadata] Mark mono_metadata_parse_method_signature external only

   Runtime should use mono_metadata_parse_method_signature_full

* [metadata] Mark mono_metadata_interfaces_from_typedef as external only.

   Runtime should use mono_metadata_interfaces_from_typedef_full

* [mini] Add MonoError arg to mono_class_get_method_generic

   Also move its declaration from metadata/class-internals.h to mini/mini.h
   since it's defined and used in mini/

* [class] Assert that mono_class_get_inflated_method never returns NULL